Sintered agglomerates and method of producing same

ABSTRACT

A sintered agglomerate consisting essentially of carbides and oxides of chromium and iron and consisting of a core covered with a dense and tough skin of metal oxides, both of which contain the same metallic elements at substantially the same proportions but contain no free carbon. This agglomerate possesses great mechanical strength and does not powder and, in addition, has properties which make it suitable for use as a starting material for producing chromium-containing ferroalloys in an electric furnace. This agglomerate is produced by pelletizing a mixture consisting of powdered chromite and a small quantity of powdered carbonaceous material, drying and preheating the resulting pellets and thereafter roasting the pellets using a combustive flame in an oxidizing high temperature zone.

United States Patent 1 1 Kusama et al. 1 Apr. 9, 1974 I SINTERED AGGLOMERATES AND 3,482,964 12/1969 lshimitsu et a1 75/3 x METHOD F PRODUCING SAME 3,661,555 /1972 Kusama et al.

3,244,507 4/1966 Linney 75/3 [75] Inventors: Fumihiko Kusama; Shoji Nakajima,

both of Chlchlbu; Mmoru Primary Examiner-A. B. Curtis Yamanaka, Tokuyama; Yoshihiro Attorney, Agent, or Firm Shernan &rsharlyloway Suzuki, Shmnanyo, all of Japan [73] Assignee: Showa Denko Kabushiki Kaisha, [57] ABSTRACT Tokyo Japan A sintered agglomerate consisting essentially of car- [22] Filed: Dec. 14, 1971 hides and oxides of chromium and iron and consisting of a core covered with a dense and tough skin of metal [21] Appl' 207996 oxides, both of which contain the same metallic elements at substantially the same proportions but con- F i A li ti P i it D t tain no free carbon. This agglomerate possesses great Dec. 30, 1970 Japan -122512 mechanical Strength and not PWder in dition, has properties which make it suitable for use as [52] US. Cl. /5 a starting material for Producing chromium'comaining [51 1m. 01 (1221) 1/16 femalbys electric furnace This agglomerme is [58] Field of Search 75/3, 5 Pmduccd by Pellelizing a mixture nsistifl8 dered chromite and a small quantity of powdered car- [56] References Cited honacelous mzliteiiial, drying and prelileatinlgl the resultm e ets an t erea ter roastln t e e ets usin 21 UNITED STATES PATENTS co rn llustive flame in an oxidizin g h1g1 temperatl ire 2,883,278 4/1959 Douglas 75/5 20% 2,915,378 12/1959 Brennan 75/5 3,153,586 10/1964 Wienert et al. 75/5 X 6 Claims, No Drawings This invention relates to a sintered agglomerate, which is conveniently usable in the production of a chromium-containing ferroalloy. The invention also relates to a method of producing this sintered agglomerate.

When chromite ore is reduced with carbon, the oxides of chromium and iron contained in the ore are reduced through an intermediate stage, wherein carbides are formed, to find the metallic state, producing a chromium-containing ferroalloy. When attempts are made to produce the chromium-containing ferroalloy by directly reducing the chromite ore with carbon in an electric furnace, great quantities of expensive electric energy are consumed, making the product very expensive. However, inexpensive fuels such as heavy oil, powdered coal and the like, can be utilized by make a sintered agglomerate wherein a substantial portion of the oxides of chromium and iron contained in the ore are reduced to their intermediate carbide form. Now, when these agglomerates are finally reduced in an electric furnace, the electric power consumption is far lower and an alloy product of lower cost can be obtained.

Consequently, chromium-containing ferroalloies are produced using the above agglomerates as the starting material or additive. These agglomerates are made in the following manner. A powdery mixture of chromite ore and a carbonaceous material is made into briquettes or pellets, and these briquettes or pellets sintered by roasting using the combustive heat of a fuel.

A typical example of the prior art sintered agglomerate is described in U.S. Pat. No. 2,883,278, which agglomerate is substantially uniform and consists essentially of oxides and carbides of iron and chromium, free carbon, and incidental impurities and has a composition such that from about 15 to 40 weight percent of the total composition is combined chromium of which from about 15 to 80 weight percent is acid soluble and the remainder is acid insoluble, from about 12 to 60 weight percent of the total composition is combined iron of which at least 85 weight percent is acid soluble and the remainder is acid insoluble, and sufiicient free carbon so that the total carbon content is at least about weight percent in excess over that necessary to provide a l to 1 mole ratio of carbon to oxygen in the aggregate, the remainder being the carbon and oxygen combined with the iron and chromium in the agglomerate. As is apparent from the foregoing description, these prior art agglomerates are substantially uniform and do not possess a tough protective coating. Accordingly, these briquettes or pellets are stored while still at a high temperature either the briquettes or pellets fuse together or the chromium or iron which have been reduced to the carbides becomes oxidized on contact with air. Further, since the agglomerate contains an excess of free carbon, its mechanical strength and ther-.

mal shock resistance is low. Therefore, it is susceptible to powdering as a result of mechanical impact during transit and thermal shock in an electric furnace, so that the gas passing over the charged layer in the electric furnace is hampered. In addition, when this agglomerate is charged as a layer on top of the coke bed layer which is the reduction zone in the electric furnace, the

agglomerate readily melts and passes through this coke bed layer without being completely reduced and falls to the bottom of the molten slag layer below the coke bed layer, i.e., to the surface of the metal layer being produced, with the consequence that the balance of the furnace conditions is lost and the continuous smooth operation of the furnace is not maintained.

In order obviate the drawbacks as described, it has been proposed to coat the surface of the agglomerate pellets with glue, starch, water glass, pitch or a slagforming constituent. These coatings, however, were disadvantageous since extra expense was required for this coating, unnecessarily increased the impurities in the product, and the coating was susceptible of stripping in transit due to difference in the properties of the coating and the interior agglomerate material.

An object of the present invention is to provide a sintered agglomerate which does not have the various drawbacks that were possessed by the conventional sintered agglomerates. Another object is to provide a method of producing such a sintered agglomerate.

According to the invention, a sintered agglomerate is provided, which consists essentially of carbides and oxides of chromium and iron and incidental impurities, said agglomerate consisting of a dense and tough skin of metal oxides and a core covered with said skin, both the skin and the core containing the same metallic elements in substantially the same proportion, said agglomerate having a composition such that the ratio of Cr/Fe is about 1.5 to about 4.0, the sum of the total Cr content and the total Fe content is about 45 percent to about 60 percent of the total weight of the agglomerate, the content of the combined carbon is less than the total Cr content multiplied by 0.099 plus the total Fe content multiplied by 0.092, about 18 percent to about 83 percent by weight of the total Cr is acid-soluble, about 51 percent to about percent by weight of the total Fe is acid-soluble, and the remainder of Cr and Fe are acid-insoluble.

Features of the sintered agglomerate of the present invention which are especially distinguishing are, its dense and tough skin its absence of free carbon.

Further, the present invention provides a method for producing the foregoing sintered agglomerate. This method is carried out in the following manner. A chromite ore and powdered carbonaceous material are mixed to produce a Cr/Fe weight ratio ranging from about 1.5 to about 4.0 and a carbon coefficient ranging from 0.5 to 1.0, this mixture is then pelletized, and the resulting green pellets are, after being dried and preheated, charged in the presence of excess oxygen into a high temperature zone of from about l,200 to about l,500C. formed by the combustive flame of fuel, to be substantially reduced while rolling.

One of the important features of the invention sintered agglomerate is its skin of metal oxides. Since this skin is formed in the roasting step by the sintering of the chromite ore itself, it is dense and tough. Further, it naturally contains the same metallic elements at substantially the same proportions as are contained in the core portion. Thus, since the agglomerate possesses a dense and tough skin of metal oxides, it is not broken or powdered by means of a mechanical or thermal shock in transit or inside the electric furnace to which it is charged. In consequence, the chromium and iron which, after having been reduced to the form of carbides, are present in the core do not become reoxidized and the gas flow is not impaired when the agglomerates are charged to the electric furnace. Thus, the invention agglomerate can, in its elevated temperature state immediately after production, be transported or be charged directly to the electric furnace for production of the alloy. When the agglomerates are charged to the electric furnace in an elevated temperature state, a saving in electric power of 500 800 K.W.H. can be made per ton of the chromiumcontaining ferroalloy produced as compared with the case where the agglomerates are charged after cooling to room temperature.

Further, the skin of metal oxides of the invention agglomerate is less fusible than the core portion. Therefore, the core portion of the agglomerates at the lowermost part of the charged layer, i.e., the core portions of the agglomerates in contact with the coke bed layer, melts first while the skin gradually melts, and the agglomerates do not pass through the coke bed layer and fall to the bottom of the furnace in an incompletely reduced state. Therefore, the material balance inside the electric furnace is maintained and not only is the continuous operation of the electric furnace made possible but the yield of the intended metallic alloy is enhanced as well.

The invention agglomerate is usually a near spherical pellet of about 30 mm in diameter, and the metal oxide skin, in order to ensure the pellets mechanical strength and the denseness required for preventing the core portion from re-oxidation, should preferably be about 0.2 mm about 2 mm thick.

The significance of the various components and their proportions of the invention agglomerate is as described below.

The above defined Cr/Fe ratio and the sum of the total Cr content and the total Fe content are proportions which can be readily obtained by roasting the pollets made by using a single chromiteore or a mixture of different natural chromite ores. Further, these proportions are within the range which is convenient for producing a chromium-containing ferroalloy which contains chromium in a desirable quantity.

Another significant feature of the invention agglomerate is that itcontains no free carbon. The amount of the combined carbon contained, i.e., the carbon present in the form of carbides, is, as previously defined, less than 0.099 X total Cr 0.092 X total Fe. While several classes of the carbides of chromium and iron are present, those of the molecular formular Cr C and Fe-,C account for practically all of those present. The amount of the combined carbon in the agglomerate is less than that calculated from these molecular formulas. The fact that no free carbon is present, plus the above described skin, contributes to the high mechanical strength of these agglomerates. In addition, since there is no free carbon inthe agglomerate, an increase can be made in the amount of additional carbonaceous material required for finally reducing the oxides of Cr and Fe in the agglomerate in the electric furnace for producing the alloy, with the consequence that the coke bed layer can be made thicker, insuring the stability of the continuous furnace operation.

The chromium and iron are respectively present in the invention agglomerate in acid-soluble and acidinsoluble forms. The term acid-soluble and acidinsoluble refer to the degree of solubility of the material in aqueous sulfuric acid. The acid-soluble forms are primarily carbides, and the acid-insoluble forms are primarily oxides. The previously defined range of the amounts of these forms of chromium and iron present in the agglomerate is subject to change depending on the composition of chromite ore used as the starting material and the quantity of mixed carbonaceous material used to make the pellets. The incidental impurities contained mean those metal oxides other than those of chromium and iron which were contained in the starting chromite ore, such as SiO A1 0 MgO and C210. The amounts of these oxides vary with the grade of the starting chromite ore.

Next, the method of producing the sintered agglomerate by means of the present invention will be described in further detail. The distinctive feature of the method is that the amount of carbonaceous material mixed with the powdered chromite ore is far less than used by conventional methods.

The chromite ore and carbonaceous material which are conveniently used have been finelly comminuted to a size of less than 147 microns, i.e., less than 100 mesh. Coke, because of its purity, is suitably used as the carbonaceous material, but anthracite or other carbonaceous materials of low'ash and volatile contents may be used slightly or as mixtures.

The amount of carbonaceous material used is limited to a carbon coefficient of 0.5 1.0. The term carbon coefficient mean a figure indicating the ratio of the amount of carbon used to the stoichiometric quantity of carbon required for reducing the total amount of oxides of chromium and iron, contained in the chromite ore, to the forms of Cr C and f e- C By mixing the carbonaceous material in a proportion smaller than the stoichiometric quantity, the pelletized agglomerates become so solid that they are hardly pulverized during the drying, preheating and roasting steps, thus not only preventing the abnormal formation of damming rings of the fused agglomcratcs which lead to the shut down of rotary kiln, used for roasting the agglomerates, but also minimizing the pulverization of the agglomerates into the size of less than 5 mm during the subsequent operation of the electric furnace, a thing which must be avoided in this latter operation, thus stabilizing the furnace operating conditions. i

On the other hand, the reason that a reduction rate which compares favorably with that of the conventional methods or is even higher can be obtained despite the lesser amount of the carbonaceous material, is the combination of the roasting temperature and oxidizing atmosphere.

If the carbon coefficient is less than 0.5, the reduction rate of chromium and iron oxides becomes low and the economical value of the sintered agglomerate suffers.

About 2 about 6 percent of binder may be added to the above noted starting materials. A small quantity of water is then sprinkled over the mixture, after which the mixture is made into pellets of near spherical form having a diameter about 10 30 mm with a granulater. Suitable binders include bentonite, water glass and waste pulp liquor, either alone or in combination. These green pellets are then dried such as by using a net type dryer. This is followed by preheating the dried pellets. The waste heat from the roasting step may be utilized in the foregoing drying and preheating steps. When the preheating is carried out using a shaft kiln, which utilizes the waste heat of the rotary kiln used for the roasting operation, there is a possibility of an oxidation loss of carbonaceous material contained in the pellets produced by steam and carbon dioxide. In order to minimize such loss, the temperature preferably should be limited less than 600C. When the preheating carried out with a gas whose content of steam and carbon dioxide is small, the preheating can be carried out with a temperature up to about 900C.

The preheated pellets are next introduced under of a supply of excess oxygen, such as air, to a zone of a temperature about 1,200 l,500C. formed by the combustive flame of a fuel, where they are roasted while being tumbled or rolled. The usual roasting kiln may be used as the roasting furnace. Suitable fuels include as heavy oil, fuel gas, powdered coal, etc.

In the present method, the carbon monoxide which is formed during the reduction of the metal oxides with the carbonaceous material is supplied with oxygen or air in order to convert the carbon monoxide into carbon dioxide by further burning thereby forming an elevated temperature oxidizing atmosphere. Thus, the pellets are subjected to the influence of this oxidizing atmosphere and an oxidation of their surface takes place to form a skin of oxides even before they arrive at the elevated temperature zone of 1,300C. This oxide skin, along with its compaction by roasting, becomes still more dense and tough at the elevated temperature zone of above 1,300C. Since this self-formed skin of oxides is dense and tough, the interior of pellets is hardly affected by the oxidizing atmosphere of the kiln. Therefore, the chromium and iron components inside the pellets, which have been reduced to carbides, are not subjected to re-oxidation. Thus, as compared with the conventional pellets having no such skin, these pellets exhibit a great improvement in the unit consumption of carbonaceous material to be added to attain the intended reduction rate. Moreover, the powdering phenomenon, which is caused by the addition of a large amount of carbonaceous material, can be controlled. In addition, as previously noted, the highly stable furnace operation can be maintained in producing alloy by completing the reduction of sintered pellets charged to an electric furnace. Further, the method has a remarkable economical advantage because the sintered pellets can be charged to the electric furnace while still in theirelevated temperature state.

Since the pellets have been thoroughly preheated, they are highly resistant to thermal shock and are hardly pulverized even when suddenly charged to an high temperature zones of about l,200 about l,500C. inside the rotary kiln.

According to the invention method, the pellets, after having been preheated, are caused to travel successively inside the rotary kiln through the elevated temperature zones of about 1,200 about l,500C. at which the reduction reaction proceeds preferably, and about l,300 about l,500C. or they are directly charged to these zones. As the pellets are exposed to a high temperature under an oxidizing atmosphere, a rapid compaction of the surface of the pellets takes place and the pellets become enveloped with a self-' formed, dense and tough oxide skin, and the resulting pellets have extremely low susceptibility to powdering upon being subjected to vigorous tumbling.

The substantial reduction of the metal oxides to carbides takes place in the 1,200 l,500C. temperature zone. The terminology substantial reduction, as here used, means that chromium oxide, in particular, is promptly and substantially reduced to its carbide. The substantial reduction of this chromium oxide is best carried out rapidly in the elevated temperature zone of 1,300 l,500C. iron oxide is more readily reduced than chromium oxide.

When these sintered pellets are withdrawn from the rotary kiln and collected in a receiving box, the sintered agglomerates of the present invention are obtained. These can be charged to the electric furnace as the starting material for producing the chromiumcontaining alloy, immediately while still in their hot state or after cooling.

Since in the case of the sintered agglomerates ob tained by the invention method, the preheated agglomerates are exposed suddenly to an oxidizing atmosphere held at an elevated temperature, as previously noted, a compaction by roasting of a high order is set up to form a dense and tough skin of oxides. Therefore, there is practically'no re-oxidation of the carbides of chromium and iron, the reduction products formed in the inside of the aforesaid sintered agglomerates, even though they are left exposed to open air after removal from the rotary kiln.

The following examples are given for illustrating the invention method as well as the sintered agglomerates obtained and their use.

Example Production of sintered agglomerates Chromite ores of varied types of grades as shown in Table l were each finely comminuted until above percent of the particles were less than microns, and 448 kg of the Phillipines chromite ore, 70 kg of the Indian chromite ore, and 482 kg of the South African chromite ore, totaling one ton, were thoroughly commingled with 189 kg corresponding to a carbon coefficient of 0.85 of powdered coke and 39.2 kg of powdered bentonite. After sprinkling about 7 percent water over this mixture, the mixture was pelleted with a pantype pelletizer while spraying additional water to obtain about 1,400 kg of pellets of spherical form of diameter about 10 30 mm. The ratio of chromium/iron in the pellets was 1.97.

These pellets were dried for 40 minutes in a net-type dryer by passing a stream of air of a maximum temperature of 250C. to reduce the residual moisture to about 0.5 percent.

The dried pellets were immediately charged to a preheater, where the preheating was carried out by passing through the preheater, the waste gas from the rotary kiln after reducing the gas temperature to about 600C. by means of a heat exchanger.

Heavy oil was used as the fuel in the rotary kiln, the highest temperature zone of which was maintained at 1,470C. while being supplied with excess air to accomplish complete combustion of fuel and generating carbon monoxide.

The foregoing preheated pellets were charged to the charging end of the rotary kiln, and, while rolling, were caused to pass through the high temperatures zone of aforesaid temperature to effect the substantial reduction thereof, after which they were caused to travel to the withdrawal end of the kiln, where they were withdrawn into a receiving box for the sintered pellets.

The composition of the so obtained sintered agglomerate was analyzed with the results shown in Table 2.

TABLE 1 GRADES OF CHROMITE ORES AND COKE COMPOSITION OF SINTERED AGGLOMERATE Molar Ratio of Composition Metallic Elements Other Properties Skin thickness Average 0.8 mm (0.5 1.1 mm) Compressive Strength Average 65 kg! pellet (43 -86 kg/ pellet) Total Cr/Total Fe 1.9% Total Cr +Total Fe 52.2%

Soluble Crfl'otal Cr 58.0%

Soluble Fe/Total Fe 86.0%

Production of alloy One thousand kg of the foregoing sintered pellets, at about 1,000C 84 kg of silica, l kg of lime stone and 95 kg of lump coke were charged as starting materials to a 4,000-KW openhearth electric furnace to produce high carbon ferrochromium, and furnace conditions were observed to obtain necessary data during operation. As the result, the following facts worthy of special mention were noted. (1) The sintered pellets in the material layer retained their original form even at a high temperature, without being powdered by its thermal shock, until they dropped into the molten reaction layer. (2) Despite the fact the sintered pellets were exposed to a high temperature in the material layer, there was almost no re-oxidation of the metallic chromium and iron that were present in the core of the pellets. (3)

Description Components (9%) Cr O FeO A1 0 MgO SiO CaO Chromite ore mined g in Philippines 49.30 17.88 11.98 15.14 3.04 0.3 Chromite ore mined in India 53.73 13.57 11.90 9.78 5.09 0.3 Chromite mixed in South Africa 44.16 24.43 15.79 10.31 2.95 0.27

carbonaceous Material Components (5%) Carbon Volatiles Ash Coke 87.0 2.2 10.8

TABLE 2 the pellets successively dropped from the lower part of the material layer into the molten reaction layer, and there was no partial fusion or the Spurting of slag around the electrodes. The electric requirement in this case was 1,850 K.W.H./ton. (When the operation was carried out in like manner using the conventional sintered pellets the electric power consumption was about 3,700 K.W.H./ton.) The metal obtained by this furnace operation is shown in Table 3. The chromium yield in this furnace operation is about 93 percent.

TABLE3 I Example 2 Production of sintered agglomerates Chromite ores of the grades shown in Table l of Example l were each finely comminuted until above 93 percent of the particles were less than 10 microns, and 603 kg of the chromite ore mined in Philippines and 397 kg of the chromite ore mined in India, totaling one ton, were commingled with 211 kg (corresponding to a carbon coefficient of 0.95 of powdered coke and 43 kg of bentonite, followed by granulation as in Example 1 into near spherical pellets of about 12 26 mm in diameter, while spraying water onto the mixture. The total weight of pellets obtained was 1,430 kg. The chromium/iron ratio of the pellets was 2.75 in this case. As in Example 1, these pellets were then dried, preheated and thereafter charged to a rotary kiln held at a maximum temperature of 1,455C., where the reduction of these pellets, while being tumbled, was carried out.

crates is shown in Table 4.

TABl .E4

COMPOSITION OF SINTERED AGGLOMERATE Molar Ratio of Composition Metallic Elements TABLE 4- Continued COMPOSITION OF SINTERED AGGLOMERATE Molar Ratio of Composition (9:)

Metallic Elements Soluble Cr/Total Cr 66.2%

Compressive strength Soluble Fe/Total Fe 75.8%

pellet pellet) Average 74 kg/ Production of Alloy 82 Kg of lump coke and 83 kg of silica were commingled with 1,000 kg of the foregoing sintered pellets of elevated temperature, and the mixture was charged to a 4,000-KW open-hearth electric furnace to produce high carbon ferrochromium.

Stable furnace operation was maintained with no spurting of gas or slag at the surface of material layer.

The composition of the resulting alloy is shown in Table 5.

Electric power consumption in this operation was,

1,920 K.W.H. per ton of alloy, which was a figure about one-half that of usual consumption. Further, the yield of chromium in the electric furnace step was about 94.3 percent.

TABLE 5 COMPOSITION OF ALLOY Components (36) Cr Fe Si C 65.9 25.2 1.4 6.4

We claim:

total weight of the agglomerate, the content of the combined carbon is less than the total Cr content multiplied by 0.099 plus the total Fe content multiplied by 0.092, about 18 percent to about 83 percent by weight of the total Cr 1s acid-soluble, about 51 percent to about percent by weight of the total Fe is acidsoluble, and the remainder of Cr and Fe are acidinsoluble.

2. The agglomerate of claim 1 wherein said agglomerate is nearly spherical and has a diameter of 10 to 30 mm.

3. The agglomerate of claim 1 wherein said skin is about 0.2 to 2 mm thick.

4. A process for producing sintered agglomerates consisting essentially of carbides and oxides of chromium and iron and incidental'impurities but containing no free carbon, comprising mixing a chromite ore and powdered carbonaceous material to produce a chromium/iron weight ratio ranging from about 1.5 to about 4.0 and a carbon coefficient ranging from about 0.5 to 1.0; pelletizing the mixture of chromite ore and carbonaceous material, drying and preheating the pellets, charging the pellets into a high temperature zone of from 1,200 to about 1,500C. formed by the combustion of a fuel in the presence of excess oxygen so that said ore is substantially reduced and rolling said pellets while being reduced.

5. The process of claim 4 wherein said ore and said carbonaceous material are mixed with about 2 to 6 percent of a binder before being pelletized.

6. The process of claim 4 wherein said temperature is from about 1,300 to about 1,500C. 

2. The agglomerate of claim 1 wherein said agglomerate is nearly spherical and has a diameter of 10 to 30 mm.
 3. The agglomerate of claim 1 wherein said skin is about 0.2 to 2 mm thick.
 4. A process for producing sintered agglomerates consisting essentially of carbides and oxides of chromium and iron and incidental impurities but containing no free carbon, comprising mixing a chromite ore and powdered carbonaceous material to produce a chromium/iron weight ratio ranging from about 1.5 to about 4.0 and a carbon coefficient ranging from about 0.5 to 1.0; pelletizing the mixture of chromite ore and carbonaceous material, drying and preheating the pellets, charging the pellets into a high temperature zone of from 1,200* to about 1,500*C. formed by the combustion of a fuel in the presence of excess oxygen so that said ore is substantially reduced and rolling said pellets while being reduced.
 5. The process of claim 4 wherein said ore and said carbonaceous material are mixed with about 2 to 6 percent of a binder before being pelletized.
 6. The process of claim 4 wherein said temperature is from about 1,300* to about 1,500*C. 